Orthogonal Frequency Division Multiple Access (OFDMA) modulation schemes have been proposed for downlink transmissions over an air interface in next generation communication systems such as 3GPP (Third Generation Partnership Project) E-UTRA (Evolved UMTS Terrestrial Radio Access) and 3GPP2 Phase 2 communication systems. In an OFDMA communication system, a frequency channel, or bandwidth, is split into multiple contiguous Resource Blocks (RBs). A grouping of multiple RBs is known as a Resource Block Group (RBG). Each RB comprises multiple, for example, 12, contiguous frequency sub-carriers that are orthogonal to each other. Under the 3GPP E-UTRA standards, a Node B then assigns the RBs to users' equipment (UEs) on a sub-frame basis, wherein a sub-frame may have a duration of one millisecond (ms). Within one sub-frame, distributed (for frequency diversity) and localized (resource block-based) transmission modes are multiplexed in an FDM manner.
That is, in a 3GPP E-UTRA communication system, UEs are assigned a Virtual Resource Block (VRB), which is a logical resource block that is associated with a same number of sub-carriers, again, 12 for example, as a RB. The VRB is then mapped to one or more RBs. One mapping scheme, known as a Localized VRB (LVRB), maps a VRB into a single RB, that is, maps the 12 sub-carriers of a VRB to the 12 sub-carriers of a corresponding RB. Localized mapping is used for Frequency Selective Scheduling (FSS), wherein transmission errors are minimized by scheduling a user equipment (UE) for a RB only where the UE is known to have a good downlink channel. Accordingly, FSS requires narrowband channel feedback from the UE, wherein the channel quality reported is specific to each RB. Reporting a CQI for each and every sub-band, or RB, may consume a significant amount of uplink system overhead, especially for OFDMA systems utilizing a 20 megahertz (MHz) bandwidth and employing as many as 100 sub-bands within that bandwidth. A second mapping scheme, known as a Distributed VRB (DVRB), maps a VRB into multiple RBs, that is, the 12 sub-carriers of a VRB are mapped to one or more sub-carriers of each of multiple RBs. Distributed mapping is used for Frequency Diverse Scheduling (FDS), wherein a VRB is distributed among multiple RBs without channel feedback or only wideband channel feedback, wherein the channel quality reported is over the whole bandwidth.
For any given Transmission Time Interval (TTI), the RBs are allocated to users based on measured channel conditions. The channel condition measurements are performed by a user equipment (UE), which UE measures channel conditions for one or more designated groups of RBs, that is, RBGs, during a measuring period, such as a Transmission Time Interval (TTI) (also known as a sub-frame) or a radio frame transmission period. The UE then reports the measured channel conditions for the RBG to a serving Node B in a Channel Quality Information (CQI) message. Based on the reported CQIs, an OFDMA communication system is able to selectively schedule the RBs over a scheduling period, typically one or more TTIs or radio frames, and further adaptively determine appropriate modulation and coding schemes for each RB during the scheduling period.
In addition, in a Multiple-Input Multiple-Output (MIMO) communication system, a UE also reports back a Precoding Matrix Indicator (PMI) for each RB. A base transceiver station (BTS), or Node B, then uses a PMI to beamform a signal for transmission to the UE via an antenna array and over an associated RB. More particularly, the BTS, or Node B, maintains a set of matrices for predistortion of signals transmitted via the antenna array. The PMI then indexes the set of matrices, indicating a set of complex values to be used to predistort a signal for transmission via the antenna array and the intervening wireless link.
Currently, CQI and PMI reporting is disjoint, that is, there is no coordination between RBs measured to determine PMIs and RBs measured to determine CQIs. The RBs measured for a CQI determination and PMI determination are determined separate from, and independent of, each other and the CQI and PMI determinations are separately reported via separate feedback channels. In addition, PMI is reported in a form of narrowband channel feedback from the UE wherein the PMI reported is specific to each RB regardless of the RBs likely to be scheduled for a UE. By contrast, CQIs may be reported in either a narrowband or wideband channel feedback form and may be reported only for the best RBs. In a system that dynamically allocates RBs every TTI, the PMI feedback is reported every TTI and can consume a significant amount of uplink capacity, and in conjunction with narrowband CQI feedback can consume an excessive amount of uplink overhead.
Therefore, a need exists for a method and apparatus that provides PMI and CQI channel quality feedback sufficient to schedule RBs and to provide optimal MIMO/beamforming weights that does not consume the overhead resulting from the separate, disjoint reporting of PMI and CQI.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.